Electricity and Magnetism PDF

ELECTRICITY AND MAGNETISM P07 - 1  Overview  The physics of electromagnetism is the combination of electric and magnetic phenomena  The basis of electronic devices: computers, TV, radio, telecommunications, lamps,…  The basis of the nature phenom...

ELECTRICITY AND MAGNETISM P07 - 1  Overview  The physics of electromagnetism is the combination of electric and magnetic phenomena  The basis of electronic devices: computers, TV, radio, telecommunications, lamps,…  The basis of the nature phenomena: lightning, auroras P07 - 2 Electric Charge and the Coulomb’s law P07 - 3  Electric Charge “Electric charge is an intrinsic characteristic of the fundamental particles which combine to make objects” It is a property that comes automatically with those fundamental particles wherever they exist.  An everyday object usually contains equal amounts of the two kinds of charge: positive charge and negative charge Electrically neutral obj:  With such an equality-or balance-of charge, the object is said to be electrically neutral; that is, it contains no net charge. P07 - 4  Electric Charge cont…. Charged objects:  If the two types of charge are not in balance, then there is a net charge. We say that an object is charged to indicate that it has a charge imbalance, or net charge. Experiment: Rubbing a plastic comb with wool (dry weather), small pieces of paper can stick to the comb. Why? P07 - 5  Electric Charge (Properties) Charged objects interact by exerting forces on one another.  Charges with the same electrical sign repel each other, and charge with opposite signs attract each other P07 - 6 Electric Forces P07 - 7  Quantization of charge Any positive or negative charge q that can be detected can be written as  SI unit of charge: C (Coulomb),  where e is the elementary charge P07 - 8  Quantization of charge  The elementary charge e is one of the important constants of nature.  The electron and proton both have a charge of magnitude e.  When a physical quantity such as charge can have only discrete values rather than any value, we say that the quantity is quantized.  It is possible, for example, to find a particle that has no charge at all or a charge of + 10e or -6e, but not a particle with a charge of, say, 3.57e. P07 - 9  Conservation of Charge  Charge can neither be created nor destroyed but it can be transfer from one object to the other.  If you rub a glass rod with silk, a positive charge appears on the rod. Measurement shows that a negative charge of equal magnitude appears on the silk. This suggests that rubbing does not create charge but only transfers it from one body to another, upsetting the electrical neutrality of each body during the process. P07 -10 Conservation of Charge Examples P07 -11 Conservation of Charge Examples P07 -12  Important Note  The term point charge refers to a particle of zero size that carries an electric charge.  The electrical behavior of electrons and protons is well described by modeling them as point charges.  Remember the charges need to be in coulombs. e is the smallest unit of charge except quarks e = 1.6 x 10-19 C So 1 C needs 6.24 x 1018 electrons or protons Typical charges can be in the µC range.  Remember that force is a vector quantity. P07 -13  Coulomb’s Law The Coulomb’s law gives the electric force between two point charges “The magnitude of the electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them” In vector form  q1q2 F12  ke 2 rˆ12 r P07 -14  Coulomb’s Law Inverse square law N  m 2 N  m 2 1 Coulomb’s constant: k  8.988x109  9.0x109  C2 2 C 2 4o 1 C permittivity of free space: o   8.85x1012 4k N  m2 P07 -15  Coulomb’s Law Two charges are separated by a distance r and have a force F on each other. P07 -16 Comparison of the Coulomb’s force and gravitational force Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) P07 -17 Comparison of the Coulomb’s force and gravitational force P07 -18 Effect of Medium on the Coulomb Force Coulomb’s Law in Vacuum P07 -19 Net force on q + P07 -20 Problem: Three charges q1 = -5 µC, q2= +10 µC and q3 = -12 µC are placed in a line as shown in the figures. Calculate the net electrostatic force on q2 due to other two charges. 6 cm 4 cm - + - P07 -21 Problem: Three charged objects are placed as shown. Find the net force on the object with the charge of -4μC P07 -22 P07 -23 P07 -24 P07 -25